Honeycomb sandwich panels constituted by a pair of prepreg skins and a honeycomb core interposed between the skins are light in weight and superior in strength and rigidity; therefore, they are used as structural members of aircraft, automobiles, etc. These honeycomb sandwich panels are generally produced by first cutting a honeycomb material into a desired shape to form a honeycomb core, covering both sides of the honeycomb core with prepreg laminate skins, mounting the resulting assembly on a forming die, if necessary, with an adhesive between the honeycomb core and the prepreg laminate, and heating the assembly under pressure for adhesion forming. In this production method, however, the edge of the honeycomb core is unable to withstand the forming pressure applied, resulting in frequent deformation (crush) of the edge. Therefore, it is desired to develop a method for producing a honeycomb sandwich panel while preventing the crush of core edge.
It is known that the crush of honeycomb core is caused by slipping of the prepregs in the skins. Japanese Patent Publication No 59-42621, for example, discloses a method for preventing the above slipping by causing projections (slipping-preventing means) of a forming die to pierce into a portion of the honeycomb sandwich panel where a lower prepreg laminate and an upper prepreg laminate are overlapping each other, inserting the projections into a press plate made of an elastic material, thereby fixing the prepreg laminates on and above the forming die. In this method using a forming die having projections, however, the projections are dangerous to an operator; the forming operation is troublesome because periodic cleaning and repair for projections are necessary; and the projections become an obstacle in sealing the forming die with a vacuum bag. Besides, there was also proposed, for example, a method for preventing the slipping of honeycomb core by fixing prepreg laminates using an adhesive tape, fixing pins, a sand paper or the like. None of these methods, however, is practical because of the problems such as increased number of steps, increased cost for forming, or the like.
A method is also known which comprises filling the edge of honeycomb core with a filler curable at room temperature, and curing the filler to increase the strength of the honeycomb core. In this method, however, widening of filling area for the prevention of crush invites increased panel weight, against the aim of lighter product weight. Moreover, because the cured filler is hard and fragile, the honeycomb sandwich panel produced may suffer cracking when the panel has a curved surface.
Further, a method was proposed which comprises providing the edge of a honeycomb core with a tapered portion to lower pressure in the forming process. The forming pressure lower than usual (about 6 kgf/cm2), however, tends to invite insufficient forming owing to pressure shortage in portions of skins where no honeycomb core is interposed. Also, an increase in the degree of tapering at the edge of honeycomb core incurs an increase in oblique dead space in a tapered portion.